Solar-driven thermochemical cycles, such as for cleaving water into hydrogen and oxygen, or the reduction of CO2 to CO, are the subject of intensive research. This is because thermochemical cycles have a high theoretical efficiency potential and, at the same time, particular requirements are placed on the processes. FIG. 1 schematically shows a corresponding cycle. In a first step, reduction of a redox material takes place. Oxygen is released thereby. The required high temperature heat can be obtained, for example, through concentrated solar radiation. The redox material reduced in this reduction step is then employed in a cleaving step. In this cleaving step, the actual cleaving of water, for example, takes place. Hydrogen is released thereby. The oxygen contained in the water is used for oxidizing the redox material. The cleavage of CO2 is similar. CO is released, and the released oxygen molecule is incorporated in the redox material. The redox material oxidized thereby is then employed again in the reduction step.
Currently, few materials are known that resist the requirements on the temperatures and reaction atmosphere and are still active. Cerium oxide is frequently used as a redox material. With it, reduction is possible at temperatures of above 1200° C. and under oxygen partial pressures of less than 1 mbar, in particular. The cleavage step usually takes place at temperatures of less than 1000° C. Thus, there is a temperature difference of more than 200° C. between the reduction step and the cleavage step, which the redox material must resist. The temperatures mentioned here are guidelines. The temperature at which the reduction and oxidation take place depends, inter alia, on the selection of the redox material, of the material to be cleaved, and of the oxygen partial pressure, so that the real temperature may both exceed or fall below the values mentioned here.
As compared to direct thermal water cleavages, the advantage of such thermochemical cycles is the lower temperature, in particular. Solar-driven thermochemical cycles usually take place in a reactor coupled with a receiver for taking up the solar energy. Receiver/reactor concepts for the solar-driven thermal cleavage of water or CO2 are described, for example, by Chue, W. C., et al., 2010, “High-Flux Solar-Driven Thermochemical Dissociation of CO2 and H2O Using Nonstoichiometric Ceria”, Science, Vol. 330, pp. 1797-1801; Ermanoski, I., et al., 2013, “A New Reactor Concept for Efficient Solar-Thermochemical Fuel Production”, ASME Journal of Solar Energy Engineering, Vol. 135, pp. 031002-1 to 031002-10; Driver, R. B., et al., 2008, “Solar Thermochemical Water-Splitting Ferrite-Cycle Heat Engines”, ASME Journal of Solar Energy Engineering, Vol. 130(4), p. 041001; Driver, R. B., et al., 2010, “Testing of a CR5 Solar Thermochemical Heat Engine Prototype” ASME, Phoenix, Vol. 2, pp. 97-104, and Lapp, J. et al., 2013, “Heat Transfer Analysis of a Solid-Solid Heat Recuperation System for Solar-Driven Nonstoichiometric Redox Cycles”, ASME Journal of Solar Energy Engineering, Vol. 135, pp. 031004-1 to 031004-11.
The integration of these receivers/reactors into an overall process could not be realized satisfactorily as yet, because partial requirements of the overall process are not or insufficiently taken into account by the mentioned receivers/reactors. However, high overall efficiencies can be realized only by taking all the requirements into account at the same time.
Particular requirements are predetermined by the redox material. The latter must be able to go through the process in a cyclic manner, i.e., the material must go through the individual process steps several times without degrading. For the reduction, high temperatures of frequently 1200° C. or more and low oxygen partial pressures are usually necessary. The actual cleaving reaction, especially of water and CO2, then takes place at clearly lower temperatures of usually less than 1000° C. A large proportion of the redox material of up to 98% often goes through the process unused, with correspondingly negative effects on efficiency. In addition, the reactions are limited by kinetics, heat and mass transport.
In the above mentioned literature, different concepts for receivers/reactors are described in which solar radiation is utilized for the reduction of redox material. These concepts follow different approaches, which depend, in particular, on the form of the redox material employed, and on the manner in which the redox material goes through the different steps of the cycle. In recent years, it has become clear that a high degree of heat recovery from the redox material is necessary for a high overall efficiency, because of the as yet low yields of redox material. Therefore, the most progressive receivers/reactors to date increasingly take heat recovery into account. In some of the concepts describes to date, heat recovery is an inherent component of the reactor. In Ermanoski, I. et al. (supra), the cleaving step has additionally been decoupled, because the cleaving reaction proceeds clearly more slowly than the reduction in the designated redox material.
In the processes described in the literature, there is a strong coupling between radiation absorption, reduction and heat recovery. Consequently, to date, no parameter window has been known with which the requirements for these subprocesses can be met satisfactorily.
From US 2013/0234069 A1 and US 2013/0004801 A1, reactors for operating solar-driven thermochemical processes are set forth. It is proposed to use liquid heat transfer media. However, this has the disadvantage that liquid media require both another solid medium for absorption in the receiver, and for heat transfer in the heat exchanger and in the reactor to transfer the heat from the liquid heat transfer medium to the solid redox material and vice versa. The two media must be in separated forms, because the reducibility of the redox material critically depends on the oxygen partial pressure in the surrounding atmosphere. The heat input into the redox material is limited by the wall between the redox material and the fluid. For operation, it is necessary that both the fluid and the wall material is stable at high temperatures, and that the wall material does not react with the redox material.
In addition, there are few liquid heat transfer media that are stable at the mentioned temperatures of clearly above 1000° C. Therefore, corresponding reactors involve particular technical claims and high safety requirements.
U.S. Pat. No. 8,420,032 B1 describes a reactor in which redox particles are transported by means of a screw. In the interior of the screw, heated redox particles fall down by gravity. A heat transfer is effected thereby from the particles in the downpipe in the middle of the screw to the upward transported redox particles.
In order to enable an efficient cleavage in a reactor, it is desirable to obtain a high yield. Thus, taking the material properties and process properties into account, as high as possible a degree of heat recovery is desirable. For the reduction of the redox material, a low oxygen partial pressure is necessary. To achieve this, a low intrinsic consumption is necessary. Further desirable is a continuous operation that is enabled despite the different reaction times for reduction, cleavage and heating. If the energy for the thermal reduction is provided by means of solar radiation, an efficient coupling thereof into the system is desirable. In addition, it should be possible to operate a reactor in a partial load range. In particular, a reactor that can be operated continuously and scalability of the process are desirable.
Although individual tasks can be accomplished with the reactors known from the literature, there has not been any reactor concept to date, especially no combination of receiver and reactor, that can meet all requirements of the overall process in a technically realizable way and satisfactorily with respect to the overall efficiency. Therefore, the object of the present invention is to provide a process and a reactor that avoid the disadvantages described in the prior art.